Foundations in Graphic Communications: Checking Color Proofs
The Halftone Dot
  Although we speak of offset lithographic plates as "planographic" or flat, the dots that make up the halftone images on them have substance and dimension. This highly magnified image shows an elliptical dot on the surface of the plate. On press, the dot will attract ink while repelling the water-based fountain solution that coats the plate in the non-image areas. The fountain solution repels ink to keep images from forming where they are not wanted.